The present invention relates to an apparatus for forming an image on an output medium.
In the case where images are output on double sides of an output medium, for example, plain paper, a one-sheet circulating system is simplest in which images are output in the order of a second page, a first page, a fourth page, a third page, . . . , while feeding the media one by one. However, the throughput, i.e., the number of sheets output per unit time is not increased since time is required for reversing the obverse and reverse of the medium, i.e., turning it over.
Therefore, an alternate circulation system is currently adopted in which an image is output on the subsequent medium when the previous medium is reversed, thereby increasing the throughput.
In the alternate circulation system, images are output to the media, respectively, in the order of a second page, a fourth page, a first page, a sixth page, a third page, an eighth page, . . . . In the alternate circulation system, generally, a medium on which an image is next output is transported to a predetermined position inside a reversing unit for reversing the obverse and reverse of the medium. The medium transported inside of the reversing unit stays inside of the reversing unit until the edition of image data to be output next is finished.
When the generation of image data for each page by an image forming apparatus is slower than the image forming speed of the image forming apparatus, an interval at which the image is output to the medium, i.e., an interval at which the image output is obtained, becomes greater than an interval at which the media can be sequentially supplied (that is to say, the throughput is decreased).
Examples of factors that cause the generation of image data to be slower than the image forming speed include:
i) much time is required for compiling, caused by complicated image data and the substantially large amount of image data;
ii) time is prolonged until the image data is transferred to the image forming apparatus, due to heavy traffic on network;
iii) an image reading speed by a scanner is slower than the image forming speed of the image forming apparatus; and
iv) a transferring speed is slower than the image forming speed of the image forming apparatus in the case where the image data is supplied via a facsimile network.
Due to these factors, the medium transported inside the reversing unit often stays inside the reversing unit for a long period of time, in comparison with the case where the media are sequentially transported.
Since a transporting path, that is, a medium transporting path inside the reversing unit includes curves in many cases, there arises a problem that the medium is curled if the medium being transported stays inside the reversing unit for a time longer than a predetermined period of time. Factors leading to the medium being curled are, particularly, small radius curves and/or the influence of heat applied to the medium in fixing.
If the medium is curled, the medium is liable to jam inside the reversing unit or along the transporting path. Furthermore, even if the medium can be free from jamming, a skew may occur on the medium transported on the transporting path or through the reversing unit, thereby raising a problem that the image output to the medium is skewed.
Here, even when the medium stays inside the reversing unit for a time longer than the predetermined time, the medium can be prevented from being curled. For example, if a region in which the medium inside of the reversing unit stays, that is, the transporting path is straight, the medium can be avoided from being curled even when the medium stays inside of the reversing unit for the time longer than the predetermined time. However, if a straight section is provided, this inevitably increases the size of the reversing unit, which increases the overall size of the apparatus, leading to increased costs.
Further, if image formation is started after storage of all the image data, it is possible to prevent the medium from remaining in the reversing unit for longer than necessary. However, providing a storage unit also increases the cost, and delays output of the first printed sheet.